The present invention relates to a process for producing 4,4'-diphenyldisulfonic acid and its monopotassium salt and, more particularly, an improved process for producing high purity 4,4'-diphenyldisulfonic acid (hereinafter referred to as DPDS) and its monopotassium salt (hereinafter referred to as DPDSK).
Recently, the range of use of heat-resistant synthetic polyester resins as engineering plastics has been increasing.
For realizing functional polymer characteristics of the engineering plastics, it is an indispensable requisite that high purity monomers be used as the starting material and contamination of the polymer with unreacted matters, isomers or inorganic matters be avoided as far as possible.
DPDS and DPDSK are important intermediates of 4,4'-dihydroxydiphenyl used as a monomeric starting material of the engineering plastics. Therefore, the development of a process for producing DPDS and DPDSK having a high quality sufficient for this purpose has been demanded.
Conventional processes have disadvantages that considerable amounts of monosulfonated compounds or DPDS isomers contained in DPDS or DPDSK seriously deteriorate quality of 4,4'-dihydroxydiphenyl obtained by the alkali fusion of DPDS and removal of these impurities in a purification step significantly lowers yields of 4,4'-dihydroxydiphenyl.
For example, a known process for producing DPDS comprises disulfonating diphenyl with 4 mol, per mol of diphenyl, of 98% sulfuric acid at a temperature in the range of 90.degree.-160.degree. C.
In this process, the reaction mixture is discharged into water after completion of the sulfonation reaction and neutralized with sodium hydroxide and then the neutralized product is filtered at a temperature of 80.degree.-90.degree. C. to obtain disodium salt of DPDS.
However, disodium salt of DPDS obtained by this process contained about 10 molar % of isomers (2,2'- and 2,4'-isomers) and about 1 molar % of diphenylmonosulfonic acid and was satisfactory for use as a starting material of a monomer used for the production of functional polymers.
The conventional process has the following essential problems. If the disulfonation reaction is carried out at a high temperature, the amount of disulfonic acid isomers is increased, though the amount of intermediate monosulfonic acids is decreased. On the other hand, if the disulfonation reaction is carried out at a low temperature, the amount of the monosulfonic acids is increased, though the amount of disulfonic acid isomers is decreased and, in addition, DPDS crystallizes out as the reaction proceeds and the reaction mixture is solidified instantaneously to make the continuation of the reaction impossible.
Another disadvantage of the conventional process is that disodium salt of DPDS is generally in the form of fine crystals and, accordingly, its filtering characteristic is quite poor and the contamination thereof with a large amount of inorganic by-products is unavoidable.
There have been proposed improved processes such as a process wherein disulfonated diphenyl is partially neutralized with sodium hydroxide and a process wherein sodium sulfate is added to the disulfonated product and monosodium salt of DPDS is salted out under an acidic condition. However, according to these conventional processes, it has been yet impossible to obtain monosodium salt of DPDS having excellent filtering characteristics or to reduce the amount of contaminants (inorganic by-products).